Portable metrology instruments such as portable coordinate measuring machine (“CMM”) arms and laser trackers are dependent on a stable mounting base to achieve their optimal repeatability and accuracy tolerances while in use. Mounting bases can take various forms including heavy rolling stands, magnetic mounts, vacuum mounts and portable folding tripods. As the measurement instruments are portable they are commonly moved to the fixture, machine, or part and set up in proximity to the fixture, machine, or part to be measured. Traditionally, portable metrology tripods have been an oversized version of a camera tripod having a center column that a clamping collar slides up and down on and this clamp is tightened to determine the opening angle of the legs and the overall working height of the tripod. It is generally preferable for a tripod's height and opening angle to not change even the slightest after being setup. Nevertheless, there still is opportunity for the tripod to relax and shift. Any amount of play or movement in the shoulder joints or strut bushings could allow the tripod to change dimensionally. Examples of these types of tripods are produced by Brunson as the M-Series and Moog as the Quickset Gibraltar. Even the slightest dimensional change can significantly affect a sensitive instrument like a laser tracker that is depending on its initial base coordinate system to not move. Once a job is started a default coordinate system is established at the base of the instrument. This internal reference is considered fixed and all points are measured relative to this coordinate system. If an operator is measuring at long distances, 80 meters for example, with a laser tracker and the base of the instrument moves even a few 0.0001″ in one or more axes the result of this is amplified over the angle and the distance to the reflector ball probe being used and would cause measurements to be off significantly more than what the tolerance might be on the part being inspected or aligned.
For this reason, some conventional tripods are tensioned to remove as much slack as possible in the joints and bushings. This helps eliminate much of the sag and drift that can be seen by just tightening the clamping collar on the center column. A deficiency associated with conventional tripod designs is that they are normally constrained to one opening angle when they are tensioned. This minimizes the number of applications where one can use a tensioned stand for industrial measurement applications. There can be benefits to the user to be able to operate the stand in a higher stance which provides both additional height and a smaller footprint. When using a laser tracker, which is a line of sight instrument, mounting the laser tracker from a higher vantage point can allow the user to measure a greater area thus minimizing the number of times one needs to move the measurement device saving time on a job. In some instances, such as measuring part of a primary power system during an outage the per-second or per-minute costs can be very significant. Another drawback of some current portable stands is that there is a requirement to have some type of spreader between the legs which is connected to each leg, one at a time like the Brunson Tetralok tripod for example.
Additionally, metrology tripods are commonly made of aluminum and carbon fiber materials and may also contain brass bushings. The combination of these materials and their joints wearing over time will cause the tripod to relax and cause the parts to fit each other loosely causing slack and movement between the parts. If a conventional hard stop is used with tensioning struts, it is possible for the tensioning struts to run out of travel, causing the tripod to lose its ability to be tensioned.
In view of the foregoing, there is a need for a portable metrology stand that can be easily and repeatably tensioned to a very stable configuration, in various positions and even after stretching, bending, and wear of the portable stand and its components.